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In translating this sentence into Latin, I am having a hard time understanding why "to the Roman Camp" is not : ad castro Romano (dative singular). I see that the footnote says it is not dative but not why.

Also the key translates it as: "in castra Romana" (dat. or acc. plural). Why is this plural, there is only one camp? I am probably making a stupid mistake and will feel very silly when someone explains this to me but I can't seem to figure it out myself.

First, you're right about the use of ad. At this point in the text you use what you have been given. As I mention in the post above, § 53 tells you that ad takes the accusative of the noun it modifies.

However, as ingrid70 duly points out to me that my declination of the word castrum is incorrect. Castrum is a neuter noun and means camp only when used in the plural. So, decline castrum:

Well, in may be ablative or accusative. Just get an instinct that ablative is stationary and accusative denotes movement. You can have prepositions such as sub and super which also take either accusative or ablative.
Compare:

1. Steven super castris volat.
2. Steven super castra volat.

The first one implies pretty much hovering I imagine. hmm...it's quite an evil thought. The second phrase shows that he flies over it and is moving past it.